The valve seats in the cylinder head of an internal combustion engine house associated valves that have the purpose of sealing the induction and exhaust conduits that lead to the combustion chamber. In order to assure the proper performance of the engine, the sealing that each valve provides must be, as a principle, absolutely airtight. However, in practice, owing to manufacturing reasons, the seating surface of the valve and that of the associated seat are subject to unavoidable shape errors that generally prevent an accurate sealing along a theoretic seating circumference, and thus generate non-contact zones where combustion air or gas leaks are formed between the conduits and the combustion chamber. These leaks determine both a drop in the performance of the engine and, owing to a faulty combustion, a rise in the amount of pollutant dispersion into the atmosphere.
In order to evaluate, before assembling a cylinder head, the extent of the sealing accuracy of the valves over their seats, it is conventional in the art to perform checkings of the geometric features of each valve seat, in particular checkings of the roundness of the conical surface of a valve seat.
There are known "plug" type devices for accomplishing geometric checkings of the conical surfaces of valve seats, disclosed, for example, in U.S. Pat. No. 2,553,129 and in German patent DE-C-2836925.
The devices disclosed in both the hereinbefore mentioned patents have a similar structure, with a central body comprising a reference surface with rotational symmetry for cooperating with the valve seat, and a cylinder-like guide stem, coaxially arranged with the body and intended for housing in the valve guide for centering the device.
The plug disclosed in the US patent has a conical reference surface and detecting means including two concentric annular rows of electric contacts in the formerly mentioned conical surface for contacting the valve seat at two distinct circumferential areas. The contacts form part of associated electric circuits that, depending on the number of contacts actually touching--in the course of the checking--the surface of the conical seat, provide indications as to possible defects, or imperfections in the valve seat.
The plug disclosed in the US patent has a structure that calls for extremely accurate machining for the reciprocal arrangement of the contacts, with respect to both the other contacts belonging to the same annular row and those belonging to the adjacent annular row.
Furthermore, as the contacts are spaced apart from one another, no complete inspection along all the points of the circumferential area is carried out.
The German patent discloses, instead, a plug with a spherical reference surface and a pneumatic detecting system with a circular slit formed in the spherical surface in correspondence with the theoretical circumference of contact. The circular slit is connected to a pneumatic circuit that leads to suitable devices that feed it and detect pressure variations.
The disclosed application requires an extremely accurate machining of the spherical surface intended for resting on the conical surface of the seat to be checked, and, above all, of the circular opening for the emission of air, the position of which must be defined with great accuracy in order to guarantee the proper operation of the plug. Thus, the structure of the plug involves high costs and inevitable inaccuracies.